(1) Field of the Invention
The present invention relates to an easily-openable heat seal lid. More particularly, the present invention relates to a heat seal lid which provides a heat seal having a high heat seal strength between a vessel proper and the lid and can resist sterilizing conditions, for example, retort-sterilizing conditions, and which can easily be opened by breaking the lid along a score line formed thereon.
(2) Description of the Prior Art
A heat seal lid having a sealing capacity and an easy-open property, for example, a so-called peelable seal lid, is known. This peelable seal lid comprises a flexible substrate such as a metal foil and a heat-sealant layer formed on the substrate. A composition formed by incorporating a wax, a tackifier and an elastomer into an olefin resin such as polyolefin, in which the seal strength is adjusted to about 1 Kg/15 mm, is ordinarily used as the heat sealant. A peelable seal lid of this type is defective in that when the content is packed and heat sterilization is carried out, the seal reliability of the lid is poor. In Official Notice No. 17 of the Welfare Ministry of Japan (enforced on Aug. 1, 1977), it is stipulated that a food vessel which is heat-sealed and subjected to heat sterilization should have a heat seal strength of at least 2.3 Kg/15 mm.
A heat seal lid sealed with such a high seal strength is disadvantageous in that peeling on the heat seal interface is manually impossible and therefore, opening is performed by a tool such as a knife or a can opener.
As the lid that can easily be opened manually without using any instrument, there is known a so-called easy-open can lid. In this can lid, an opening portion is defined by scores on a lid formed of an aluminum sheet, a rivet is formed on the opening portion and a pull ring is secured by this rivet of the opening portion, and this can lid is double-seamed to a flange portion of a can body.
This easy-open can lid can resist heat sterilization under pressure and has an excellent easy-open property. However, this can lid is defective in that a large quantity of expensive metallic aluminum should be used as the material, the processing steps are complicated and troublesome and the cost is considerably higher than that of the above-mentioned flexible heat seal lid.
Moreover, vessels to which this easy-open lid can be applied are limited to those having a high heat resistance, for example, cans. For example, when this easy-open lid is double-seamed to a plastic cup-like vessel formed by drawing, it is impossible to form a seal having a high reliability, and when the cup-like vessel is subjected to hot packing or retort sterilization, since the flange of the cup is softened at high temperatures adopted for such treatment, formation of a reliable seal becomes more difficult.